The exploitation of undersea oil resources has led to the development of various types of platform and in particular of permanent platforms provided with columns which stand on a foundation resting on the sea bed.
The deeper the sea, the higher the cost of these structures.
Further, the above type of platform cannot be envisaged when the depth of the sea exceeds several hundreds of meters.
That is why normally-floating platforms have been invented which are deliberately kept submerged so as to reduce the effects of the swell (pounding, rolling and pitching). They are kept partially submerged by means of rods each having one end fixed to the sea bed and the other end fixed to the platform.
The immersion process is illustrated in FIGS. 1A to 1F. FIG. 1A illustrates a fragment of the platform 1 floating on the sea whose surface is referenced 2 and whose bed is referenced 3. The rods which support the platform are assembled end to end in a passage 4 in accordance with a technique which is well-known in the field of oil well drilling so as to constitute a set of rods 5.
The set of rods ends in a male connector 6 designed to co-operate with a female connector 7 fixed in a solid mass 8 fixed in the sea bed (see FIG. 1C).
Said male connector is of the type equipped with fixing fingers which open automatically like a corola when enough pressure is applied to their ends; the female connector has a step which co-operates with the fingers to make the two components fast together.
When the set of rods has been assembled (FIG. 1A), it is raised (FIG. 1B) so that its bottom end is at a higher level than the solid mass, with a margin which takes pounding into account to avoid the danger of shocks.
The set of rods is moved to a position above the female connector (FIG. 1C) and is then lowered (FIGS. 1D and 1E) so as to insert the male connector in the female connector with sufficient pressure to open the fixing fingers 6A of the male connector.
Lastly, the platform and the set of rods are moved relative to each other so as to partially immerse the platform which is then practically prevented from moving and made insensitive to the movements of the sea.
At the end of the partial immersion step, the top end 5A of the set of rods is made fast to the platform.
The preceding description relates to a single set of rods: it is obvious that the platform is equipped with a plurality of sets of rods operated simultaneously and in synchronism.
For example, a plurality of sets of rods are placed at each of the four corners of a square platform.
Preferred embodiments of the invention provide a device which controls the movements of a set of rods in a positioning sequence such as the one which has just been described.
It should be observed that the problem is harder to solve than it seems. In particular, the device must be able to cause movements in a system whose apparent weight can vary over a wide range depending on the stage of the operation (e.g. from about one hundred tons to about one thousand tons), while being sensitive to weights of about ten tons, in particular to ensure proper and reliable operation of the automatic connector.
Further, the device must be able to cause movements of the set of rods over an amplitude of about ten meters yet without requiring special jacks to be manufactured which do not fall within manufacturers' usual production standards for jacks.
Preferred embodiments of the invention further provide a device in which the rods can be positioned with damping means which permanently damp shocks and the effects of the swell.